The present invention relates to methods for manufacturing semiconductor devices, the methods include processes of thermally-oxidizing surfaces of semiconductor substrates made by silicon carbide (SiC) as body material, or a semiconductor layer made by SiC as body material, configured to form a silicon dioxide film (SiO2 film), and semiconductor manufacturing apparatus, which can be used for the methods.
As an earlier SiC semiconductor device, a MOSFET or a MOS capacitor for example has been known that uses a semiconductor substrate made by SiC as body material (hereinafter referred to as “SiC substrate”), or a semiconductor layer made by SiC as body material (hereinafter referred to as “SiC layer”). As a gate insulating film for MOSFET or an insulating film for MOS capacitor, an SiO2 film has been used. The SiO2 film has a high insulating characteristic due to the behavior that the SiO2 film has a wide band gap and the SiO2 film forms a large energy barrier height at the interface between the semiconductor and metallic film. Thus, various applications using the SiO2 film have been expected in the SiC semiconductor device.
Methods of forming a SiO2 film on a surface of an SiC substrate or an SiC layer include, for example, a thermal oxidation method using the exposure of a surface of the SiC substrate or the SiC layer, being heated to a high temperature, to oxidizing ambient, a chemical vapor deposition (CVD) method by introducing and pyrolyzing raw material gas such as the organic silane-based gas on the surface of the SiC substrate, or a physical vapor deposition (PVD) method such as a sputtering method. Among these methods of forming the SiO2 film, the thermal oxidation method is advantageous from the viewpoint of the insulating characteristic such as leakage current or the interface level.
However, the formation of an SiO2 film on the surface of an SiC substrate or an SiC layer based on the thermal oxidation method is reported that interface defects unique to the interface between SiO2 and SiC are generated. These interface defects cause the interface between SiO2 and SiC to have a very poor characteristic. Therefore, in the case of an SiC-MOSFET, which has a gate insulating film made by an SiO2 film formed by the thermal oxidation method on the SiC substrate or the SiC layer, because the channel mobility is much lower than the speculated value based on the bulk mobility of the SiC, the practical use of thermal oxide film in a transistor is prevented. Thus, since the SiC-MOSFET has a very-low channel mobility, the on-state resistance value (Ron) is much higher than a value theoretically estimated based on the physical property value.
In the case of four-fold periodicity hexagonal silicon carbide (4H-SiC) in particular, regardless of the fact that the bulk electron mobility is about 900 cm2/Vs, the MOSFET in which a gate oxidation film is formed by a general thermal oxidation method such as dry oxidation has a very-low channel mobility of about 0 to 5 cm2/Vs. This is presumably caused by the high interface level density in the vicinity of the conduction band of the channel region.
General methods of improving the interface characteristic between SiO2 and SiC include a known method according to which a surface of an SiC substrate or an SiC layer is oxidized in an atmosphere including oxygen (O2) to subsequently supply gas containing dinitrogen monoxide (N2O) or nitric oxide (NO) as POA (Post Oxidation Annealing) (see JP2004-511101A1). In this case, it is considered that the oxidization and nitridation simultaneously so that nitrogen can contribute to the termination of the dangling bond of the interface during the oxidation process, thus achieving an effectiveness of decreased interface level density.
Another method has been suggested to form a thermally-oxidation film on the surface of an SiC substrate or an SiC layer in O2 or a water vapor atmosphere at a temperature from 1100 degrees C. to 1200 degrees C., and then decrease the temperature of furnace in inert gas to so as to minimize the interface level density (see JP2003-31571A1).
However, in the case of the method according to JP2004-511101A1, by which ambient gas containing N2O or NO is used, there is a concern that the nitrogen-derived charge trapping center is generated in an oxidation film and the oxidation film has a deteriorated long-term reliability, regarding the electric stress to the oxidation film, for example. In the case of the method according to JP2003-31571A1 to decrease the interface level density by lowering temperature in inert gas on the other hand, the effectiveness to decrease the interface level density is insufficient.
In view of the above problems, it is an objective of the present invention to provide a manufacturing method of a semiconductor device according to which the interface level density between SiO2 and SiC can be decreased when the surface of an SiC substrate or an SiC layer is thermally-oxidized to form an SiO2 film, and a semiconductor manufacturing apparatus, which can be used for the manufacturing method.